The object of the invention is a method of condensing and/or polymerising sulfonated lignin containing materials by means of which the viscosity, molecular weight and the efficacy of the materials may be substantially increased.
According to the invention polymers based on sulfonated lignins are being produced in a substantially dry state and at relatively low temperature and the end products thus obtained exhibit remarkably increased viscosities. The novel and surprising feature of the invention is not so much the increase in molecular weight as the simultaneous pronounced viscosity increase.
A main source of sulfonated lignin are residual pulping liquors of wood, straw or bagasse. In the sulfite pulping process the lignocellulosic material is digested with a bisulfite or sulfite salt solution, whereby a sulfonated lignin containing solution is formed, which is commonly referred to as spent sulfite liquor. In other pulping processes a spent liquor is obtained which does not contain sulfonated lignin. The lignins thus obtained can be sulfonated by additional treatment whereby sulfonated lignins, also suitable for the preparation of the material according to the invention, are obtained.
It is known that the sulfonated lignin containing products obtained by the above mentioned processes differ substantially as to their composition and the chemical structure of their various constituents. Furthermore, many of the commercially available products are prepared by enriching the sulfonated lignin material or by chemically modifying one or more of the components of the spent liquor. Although the composition of such preparations may vary in many different ways, they still constitute sulfonated lignin containing material also known as lignosulfonates. As a group of compounds the lignosulfonates are invariably polydisperse and are described with particular thoroughness in the book `Lignins` edited by K. Sarkanen & C. Ludwig, Wiley-Interscience.
Of the millions of tons of sulfonated lignin produced annually only a small fraction is currently finding industrial utilization. Furthermore, utilization for the most part is confined to whole spent sulfite liquor exhibiting viscosities of several hundred cP as 50% solutions at room temperature and containing sulfonated lignin matter that for more than 60% falls into a molecular weight range of under 5000.
Products of higher average molecular weight and viscosities are being offered on the market as premium priced specialty items. These compounds are attracting considerable attention because of their enhanced usefulness in - for instance - such diverse materials as dispersion agents for organic dyestuffs, precipitation agents for soluble protein or as additives or extenders for glues.
For the most part these relatively high molecular weight lignosulfonates are derived by costly fractionation of sulfite spent liquor. However, spent sulfite liquor solids have also been polymerised by a method described in Canadian patent 436 469. Said patent discloses acid polymerisation of lignosulfonates in an aqueous medium to produce products claimed useful for oil well drilling muds and other applications. Accordingly, highly corrosive solutions warranting very special and costly manufacturing equipment need to be handled for prolonged periods of time at temperatures up to 180.degree. C and pH values less than 1. A further drawback severely limiting any polymerisation technique in solution is the fact that the reaction is accompanied by a simultaneous increase in the viscosity of the medium. For the reasons of technical feasibility, polymerisation in concentrated, i.e. already very viscous solutions is therefore limited to minor increases in the molecular weight while polymerisation in the dilute state would prove excessively costly and run the risk of concurrent hydrolysis to products having a lower molecular weight or being more polydisperse as they contain hydrolysed matter in addition to condensed matter.
A method for heating lignin sulfonates in the dry state is described in U.S. Pat. No. 3 476 740. Accordingly a sulfonated lignin containing material is subjected to heat treatment between temperatures of 200.degree. and 330.degree. C to produce products useful in drilling muds and for dispersing applications. Because the preferred temperature range employed, 230.degree. to 270.degree. C, is above the ignition temperature of the treated material and the energy input is is sufficiently large to cause homolytic and other breakdown of the sulfonated lignin material, it is found necessary to first stabilize the lignin sulfonate by oxidation. Similarly, German Pat. 1769903 also discloses a method for heating lignin sulfonates at temperatures above 150.degree. C leading to useful products exhibiting lower viscosity than the starting material.
By contrast, the present invention aims at producing substantially water soluble condensation products of sulfonated lignins which are commercially attractive whereby the method is sufficiently mild as to give rise to a minimum of side reactions.
The characteristics of the invention appear from the appended Claims.